Why there are so many different function prototype for one function

I look the reference of std:istream:get and found that there are many function prototypes.
public member function
int get();
istream& get ( char& c );
istream& get ( char* s, streamsize n );
istream& get ( char* s, streamsize n, char delim );
istream& get ( streambuf& sb);
istream& get ( streambuf& sb, char delim );

then how do I know that which one function is called when calling get?
Thx in adhvance.
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